1. Field of the Invention
The present invention relates to an image forming device according to an electro-photographic method by which a plurality of images formed on photosensitive bodies by a plurality of optical beams are overlapped and output as a single image. More particularly, the invention relates to an image forming device such as a laser-beam color printer and a digital color copying machine. In these machines, the photosensitive bodies are scanned with optical beams emitted from each light-emitting sections and exposed to the beams by lighting each of the light-emitting sections in a multi-beam laser, based on image information.
2. Description of the Related Art
Recently, a multi-colored image forming device such as a laser-beam color printer has been demanded to achieve higher speed and better image quality by lower cost than previous models.
A tandem method has been known as a method for increasing the speed of the image forming device. According to the method, photosensitive bodies which are individually installed for each color are scanned with optical beams to form images for each color and a plurality of images are overlapped on a transferring medium to form a color image.
Conventionally, for example, a device has been disclosed in the Japanese Patent Application Laid-Open (JP-A) No. 63-271275 as this kind of a multi-colored image forming device.
As disclosed in the above application, four photosensitive bodies are arranged corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K) to form a four-colored image. Optical scanners for optical-beam scanning are provided for each photosensitive body. In this method, a high speed image forming device is realized by simultaneous operations of image forming for each of four colors and the four optical scanners have the same configuration.
In this method, fine adjustment of optical components such as mirrors provided in each optical scanner or the optical scanners themselves is executed to correct deviations between positions of optical beams for each of four colors to be overlapped.
A device is disclosed in the JP-A No. 59-123368 is one example in which optical beams enters different reflecting surfaces of one rotating polygonal mirror in order to reduce the number of components.
In this method, each of the plurality of optical beams is configured to enter onto different reflecting surfaces of the rotating polygonal mirror. Optical beams after reflection and deflection by the rotating polygonal mirror are reflected and deflected in different directions to the mirror respectively.
The optical beams reflected and deflected in different directions at the rotating polygonal mirror have a different direction as a main scanning direction on the photosensitive body from each other.
In this method, fine adjustment of optical components such as mirrors provided in each optical scanner causes fine adjustment of positions of optical beams to which the photosensitive body is exposed. Accordingly, deviations between positions of optical beams for four colors to be overlapped are corrected.
A device disclosed in the JP-A No. 9-184991 is one example in which a plurality of optical beams enter onto one rotating polygonal mirror. Components for optical scanning systems are commonly used.
In this method, the optical beams enter the same reflecting surface of the mirror. Optical beams reflected and deflected by the rotating polygonal mirror are reflected and deflected in the same direction to the rotating polygonal mirror respectively.
All the optical beams reflected and deflected in the same direction by the rotating polygonal mirror have the same direction as the main scanning direction on the photosensitive body.
In this method, fine adjustment of optical components such as mirrors provided in each optical scanner causes fine adjustment of positions of optical beams to which the photosensitive body is exposed. Accordingly, deviations between positions of optical beams for four colors to be overlapped are corrected.
A method in which a surface emitting laser with a plurality of light-emitting sections arranged in two dimensions is used as a light source has been known as a method by which a high image quality image forming device is obtained.
A device disclosed in the JP-A No. 2001-215423 is one example in which a surface melting laser with light-emitting sections arranged in two dimensions is used as a light source. One surface emitting laser is provided with 36 light-emitting sections.
A high-density optical writing with a density of 2400 dpi can be realized by scanning and exposing the photosensitive body with 36 optical beams emitted from the surface emitting laser at the same time.
In a device disclosed in the JP-A No. 2001-215423, timing for lighting each of the light-emitting sections in the main scanning direction is controlled. This leads to that amounts of the following offset are corrected at image forming, since in surface emitting lasers, a plurality of light-emitting sections are arranged offset in the main scanning direction.
A starting position for writing an image in the main scanning direction is controlled by provision of a synchronization optical sensor outside an image area. Deviation of a starting position for writing an image which deviation is caused by an exposed image with two dimensional broadening is prevented by using only one row of the light-emitting sections (6 sections) to be lit on the surface emitting laser. The row of the sections is arranged in the sub-scanning direction among 36 light-emitting sections.
As described above, in optical systems using the surface emitting laser with the light-emitting sections which are arranged in two dimensions, a plurality of optical beams emitted from the surface emitting laser have two axes in two directions. Assuming that the optical axis is a normal life, the plurality of optical beams are arranged in two dimensions on a plane defined with the two axes.
Let us assume that the X axis is a main scanning direction and the Y axis is a sub-scanning direction. In FIG. 4, for example, where 36 light-emitting sections are aligned and arranged in the sub-scanning direction by 6 sections, the 36 light-emitting sections have six coordinates on the X axis and have 36 coordinates on the Y axis.
In the optical systems which use multi-beam lasers with such a plurality of light-emitting sections arranged in two dimensions, the direction of either axis of two axes is reversed, depending on returning directions, when there is in the optical systems a mirror which returns optical beams.
That is, when there is an optical system mirrors 100 and 102, which return optical beams in the main scanning direction, as shown in FIG. 8, the direction of the optical beams in the X direction is reversed before and after passing (reflecting at) the mirrors 100 and 102.
When mirrors 104 and 106 are in an optical system, which return optical beams in the sub-scanning direction, as shown in FIG. 9, the direction of the optical beams in the Y direction is reversed before and after passing (reflecting at) the mirrors 104 and 106.
A method by which the tandem configuration and the two-dimensional multi-beam laser such as the surface emitting laser are applied in a multi-colored image forming device as described above is effective for a high speed and high image quality multi-colored image forming device. However, there have been the following problems when the configuration and the laser are installed at the same time.
When the two-dimensional multi-beam laser is used in the light-emitting section, and when the optical systems corresponding to a plurality of colors in the multi-colored image forming device are different from each other, arrangements of a plurality of optical beams on the photosensitive bodies become different for each color in some cases.
when the beam arrangements on the photosensitive bodies become different from each other, the following problems occur.
In the first place, a case in which beam arrangements in the main scanning direction are different from each other will be explained.
In the two-dimensional multi-beam laser, offset in the main scanning direction at light-emitting sections are controlled to be canceled and, then, starting positions for writing images in the main scanning directions have the same position.
When the starting positions for writing are different from each other for each color, offset control is required for different positions and leads to increase the production cost.
In order to control the starting positions for writing images, optical beams for synchronization detection which are lit on synchronization sensors are also different from each other. Accordingly, control means are required for different beams to increase the production cost.
When the directions of the sub-scanning directions are different from each other on photosensitive bodies, to reverse the direction of image data to a sub-scanning direction beforehand, which is input to a multi-beam laser, is required for aligning the directions of images in the sub-scanning directions. Accordingly, another means for reversing image data is required to increase the production cost.
FIG. 10 shows one example in which two-dimensional multi-beam lasers are applied to an optical system on an image forming device according to the JP-A No. 59-123368. Two light sources 108 and 110, and returning optical mirrors 112 and 114, corresponding to the light sources 108 and 110, in the optical system are extracted for explanation.
Two axes of two-dimensional optical beams emitted from the light sources 108 and 110 have the same direction. A reference numeral 115 indicates a rotating polygonal mirror.
With regard to the directions of the two-dimensional optical beams on photosensitive bodies 116 and 118 to be exposed respectively to the optical beam, the X axes (axes in the main scanning direction) both correspond to the main scanning directions of optical systems (the axial directions of the photosensitive body 116 and 118). The sub-scanning directions are opposite to each other regarding the rotation directions of the photosensitive bodies 116 and 118.
Thus, in order to align the directions of the images which are made on the photosensitive bodies 116 and 118 by the optical system in FIG. 10 after exposing, the direction of an image signal to be input to either of the light sources 108 or 110 must be reversed to the sub-scanning direction.
Reversing images additionally requires changing a control board of a surface emitting laser or an image control board. This means that the number of different parts will increase and the number of common parts will decrease. As a result, the production cost will increase.
When a common signal is used as an image signal, that is, a common board for image control systems is tried, two kinds of surface emitting lasers are required. One kind of the laser is that having directions of the two axes shown in FIG. 10 and, another kind is that only the Y axis (the axis in the sub-scanning direction) is reversed for surface emitting lasers as the light sources 108 and 110. As a result, a common light source is unpractical and higher cost would be required in any way.
The two-dimensional multi-beam laser is applied to an optical system in an image forming device as disclosed in the JP-A No. 9-184991. Light sources 120 and 122, and returning mirrors 123, 124 and 126 in the optical system, through which optical beams from the light sources 120 and 122 pass, are extracted from the disclosure for explanation.
In FIG. 11, two mirrors after a rotating polygonal mirror are omitted from the image forming device disclosed in the JP-A No. 9-184991. Images are reversed and returned to the original ones with two mirrors which optical beams reflect in the same direction. This is substantially the same as that shown is no mirror. Accordingly, the two mirrors are omitted in the drawing. A reference numeral 127 is a rotating polygonal mirror.
As shown in FIG. 11, the direction of two axes of two-dimensional optical beams emitted from the light sources 120 and 122 are the same.
The two axial directions of photosensitive body drums 128 and 130 to be exposed to the individual optical beams are opposite to each other regarding the main scanning direction of the optical beams (the axial directions of the photosensitive body drums 128 and 130).
Thus, in order to align the orientations of images exposed to the photosensitive body drums 128 and 130 by the optical system in FIG. 11, reversing an image signal, which is for inputting to either of the light source 120 or 122, will be necessary to the main scanning direction at a proper time.
Modification of a control board of a surface emitting laser or an image control band will be necessary in order to reverse the image signal at the proper time. This means that the number of different parts will increase and the number of common parts will decrease. As a result, the production cost will increase.
When a common signal is used as an image signal, that is, common image control boards are tried to be employed, two kinds of surface emitting lasers corresponding to light sources 120 and 122 will be needed. One kind is the one having two axial directions shown in FIG. 11. Another is that only the X axis (the axis in the sub-scanning direction) is reversed. This means that the number of different parts will increase and the number of common parts will decrease. As a result, the production cost will increase.
In a method, by which optical scanners for optical-beam scanning are installed for each photosensitive body, as disclosed in the JP-A No. 63-271275, the directions of the two axes of the two-dimensional optical beams become the same on four photosensitive bodies when four optical scanners have the same configuration. In this case, the above-described problems will not arise.
There are some conditions where optical scanners need to be modified due to inner layout of an image forming device, and black images are high-speed output for increasing output productivity of monochrome images. When the system disclosed in JP-A No. 63-271275 is under these conditions, problems similar to the above-described problems were observed.